In a background art, there is widely known a developing method for developing an electrostatic latent image formed on a surface of a photosensitive drum by a developer agent (toner) (for example, see JP-A-2004-037779).
According to the developing method, a toner is charged by friction by a supply roller and a layer thickness restricting blade and thereafter, adhered to a surface of the developing roller in a state of being formed into a thin layer. Further, by generating an electric field by applying a developing bias voltage to the developing roller, the charged toner is made to adhere to a portion formed with the electrostatic latent image. Further, the toner adhered to the electrostatic latent image portion is transcribed onto a sheet and a desired image is formed on the sheet by bringing the toner into press contact therewith by a fixer.
On the other hand, the toner adhered onto the developing roller which is not used for development is returned again to a toner hopper and is agitated with the toner remaining in the toner hopper by an agitator. Further, when the development is carried again, the toner is charged by friction by the supply roller and the layer thickness restricting blade and thereafter adhered to the surface of the developing roller.
In this way, the toner is repeatedly subjected to successive cycles of friction charging->adhering to the developing roller->returning to the toner hopper->friction charging and therefore, with an increase in a number of times of development, the toner is gradually deteriorated to be difficult to be charged, since in the above-described developing method, development is carried out by utilizing operation of a force which an electric charge (charged toner) receives from the electric field, when an amount of charging the toner is reduced, the development cannot excellently be carried out and an image quality is deteriorated. In order to prevent the deterioration in the image quality beforehand, prior to deteriorating the toner, the toner and therefore, the developing cartridge (having the developing roller, the supply roller, the layer thickness restricting blade, the toner hopper and the like) may be interchanged by informing the deterioration of the toner to an operator and in order to realize the interchange, for example, a counter for counting a revolution number of the developing roller may be provided at an apparatus main body and when the revolution number reaches a predetermined rectified number of times, a timing of interchanging the toner may be displayed.
However, according to the above-described configuration, the counter is needed and therefore, a number of parts is increased and when the counter is failed, even when the developing roller reaches the rectified number of times, the timing of interchanging the toner is not displayed and the operator continues printing by the deteriorated toner.